This invention relates generally to gas flow control apparatus and, more particularly, to apparatus for controlling the relative rates of flow of gases through ducts. It is not uncommon in gas flow systems to require the maintenance of a particular value for the relative rates of flow of gases through ducts. For example, in ventilation systems, there is often a need to maintain a desired relationship between the rates of flow of exhaust air and intake air. For example, where a change in the flow rate of outgoing exhaust air occurs, it is desirable and often necessary to accordingly regulate the rate of flow of the intake air.
In this connection, where the intake and exhaust air supplies in a ventilation system are used in conjunction with a heat recovery unit, it is important to maintain the relative rates of flow of the air through the intake and exhaust ducts. More particularly, such heat recovery units typically include a housing divided into intake and exhaust portions by a thermal transfer sheet whereby exhaust air transmitted through one portion will transfer heat through the thermal transfer surface to the incoming intake air directed through the intake portion of the unit. It is not unusual for condensate to form in such heat recovery units which often freezes in cold weather thereby affecting the relative flow rates in the exhaust and intake ducts in such cases, as discussed below it is desirable to provide an apparatus for maintaining the relative flow rates of the desired level.